The present invention relates generally to a process for operating a plasma arc torch, and more particularly to a start up sequence and a shut down sequence that significantly extends the life of the electrode and nozzle.
The operation of conventional plasma arc torches is well understood by those in the art. The basic components of these torches are a body, an electrode mounted in the body, a nozzle defining an orifice for a plasma arc, a source of ionizable gas, and an electrical supply for producing an arc in the gas. Upon start up, an electrical current is supplied to the electrode (generally a cathode) and the pilot arc is initiated in the ionizable gas typically between the electrode and the nozzle, the nozzle defining an anode. Then, a conductive flow of the ionized gas is generated from the electrode to the work piece, wherein the work piece then defines the anode, and a plasma arc is thus generated from the electrode to the work piece. The ionizable gas can be non-reactive, such as nitrogen, or reactive, such as oxygen or air.
A significant problem with conventional plasma arc torches is wear of the electrodes and nozzles. Typically, the electrodes include a hafnium or a zirconium insert. These materials are desired for their material properties, but are extremely costly and require frequent replacement.
It has been found that a significant percentage of the electrode wear and damage occurs during shut down of the torch. It is believed that on cut off of electrical current to the electrode, wear results from a complicated interaction between molten surfaces of the electrode and the pressurized flow of the plasma gas through the nozzle. The phenomena is also described in U.S. Pat. No. 5,070,227 which is incorporated herein by reference.
One form of electrode wear includes the formation of an oxide such as Hafnium oxide or Zirconium oxide. As set forth in the Applicant""s prior U.S. Pat. No. 6,093,905 which is incorporated herein by reference, the oxidation of the electrode is a major contributor to electrode wear and loss. Applicant""s prior U.S. patent describes a shut-down protocol for a plasma arc torch which is varied over the life of the electrode so as to beneficially remove accumulated oxide material from the electrode.
It is also understood that the electrodes, and particularly the inserts, have a limited number of cycles or xe2x80x9cpiercesxe2x80x9d. A xe2x80x9cpiercexe2x80x9d refers to the starting up and initial cutting or piercing of the arc through a work piece. For each pierce there is obviously a prior shut down of the torch and an associated start up sequence. Plasma torches utilizing conventional shut down methods and operating above 100 amps have an electrode life of generally between about 400 to 800 pierces.
The industry is constantly seeking methods for improving the plasma torches, and particularly for extending the life and improving the wear characteristics of the electrodes. The present invention concerns just such an improved method.
It is, therefore, a principal object of the present invention to provide a process for operating a plasma arc torch, particularly on shut down, that significantly reduces oxidation formation of the electrode element, and loss of molten element material and thereby reduce electrode element erosion and eliminate nozzle damage.
An additional object of the present invention is to provide a process for shutting down conventional plasma arc torches that can be readily practiced by conventional torches with relatively minor modifications.
Additional objects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention.
In accordance with the objects and purposes of the invention, a process is provided for operating a plasma arc torch on shut down. The process operates on the principles of substantially reducing the oxidation of the electrode and further eliminating loss of molten electrode material during the shut down sequence in order to substantially increase the life of the electrode. It has been found by Applicants that, through practice of the present invention, electrode life of conventional torches can be extended as much as four-fold.
It is another aspect of the present invention to provide a new and useful start up sequence which controls the gas flow relative to the arc current. Applicant""s start up method eliminates nozzle damage, reduces electrode erosion, and maintains a stable arc. The improved start up methodology may use conventional gas supply and control means. The start up method has the additional advantage of reducing damage to torch components caused by molten metal during piercing of the work piece during start up. The start up sequence allows a more rapid penetration through material and thus minimizes molten metal blow back that may otherwise damage the torch frontend components.
The invention will be described in greater detail below through use of the appended figures.